A DirectUpload may fail for several reasons, and return many types of
errors (string, xhr response, Error objects, etc).
For convenience, wrap all these errors in a FileUploadError object.
- It makes easier for clients of the Uploader class to handle errors;
- It allows to propagate the error code and failure responsability.
When the authenticity token is invalid, the creation of the blob before
the direct upload returns a 422.
In that case, the token will never become valid again, and it is useless
to try again. Don’t show the "Retry" button in this case.
NB: of course the real fix is to understand why the authenticity token
is so often invalid – but this will be for later.
